Suspensions for vehicles are structural devices used for suspending a vehicle body and absorbing shocks from the road during the operation of a vehicle, thereby preventing the shocks from being applied to the vehicle body and to passengers. Thus, the suspensions must be designed such that they can attenuate shocks from a road and make passengers feel comfortable despite the shocks, and improve steering stability, determined by the ground contact force of tires during running of vehicles. Another important factor to be considered while designing suspensions is that the suspensions must maintain desired stiffness and desired durability despite the repeated application of shocks from roads thereto. Deformations or cracks formed in the suspensions may impose fatal effects on vehicle stability, and thus the durability design of the suspensions plays an important role in the functional design of the suspensions.
Particularly, a torsion beam suspension, typically used as a rear suspension of a small-sized vehicle, must be designed to have high durability because a torsional load is repeatedly applied to a torsion beam of the suspension. In the torsion beam suspension, the cross-sectional shape of the torsion beam plays an important role in the durability performance of the beam. The cross-sectional shapes of torsion beams may be variously designed according to the different characteristics of vehicles. However, in the initial stage of designing a torsion beam, the cross-sectional shape of the torsion beam must be determined in relation both to the roll stiffness and to the roll strength of a vehicle using the torsion beam, and thus it is required to carefully study the roll stiffness and the roll strength.
In other words, the torsion beam of a rear suspension, which couples a left wheel and a right wheel together, is an important element in maintaining the stiffness of the suspension and in determining the dynamic characteristics of the suspension during the operation of a vehicle. Thus, the torsion beam must be designed such that it has appropriate roll stiffness, determined by the weight of the vehicle, so as to resist torsional deformation and bending deformation, which take place when the left wheel and the right wheel execute respective motions in opposite directions. Further, because normal stress and shear stress are concentrated on the torsion beam, it is required to design the torsion beam such that the beam has appropriate roll strength and has fatigue resistance determined in consideration of running-induced fatigue.
Hereinbelow, the construction and problem of a prior art torsion beam suspension will be described with reference to FIG. 1, which shows a suspension equipped with a conventional plate-type torsion beam. The prior art torsion beam suspension, typically used as a rear suspension in a small-sized vehicle, comprises two trailing arms, which are left and right trailing arms 2 coupled together by a plate-type torsion beam 3, and a bush sleeve 1, which is provided at the front end of each of the two trailing arms 2 and pivots on a vehicle body using a rubber bush. Further, both a spring seat 4 for supporting a suspension spring thereon and a damper bracket 5 for supporting a shock absorber are mounted to the inner side of the rear end of each of the two trailing arms 2. Both a wheel carrier 6 and a spindle plate 7 for holding the rear wheels of a vehicle are mounted to the outer side of the rear end of each of the two trailing arms 2. The above-mentioned bush sleeves 1, trailing arms 2, spring seats 4, damper brackets 5, wheel carriers 6 and spindle plates 7 form basic elements constituting the torsion beam suspension.
The conventional plate-type torsion beam 3 is typically produced using a thick iron plate having a thickness of about 4˜6 mm through pressing such that the beam 3 has an open cross-section in a shape of ⊃, ⊂, , <, >, etc. The plate-type torsion beam 3, having the above-mentioned open cross-section, has low stiffness and low strength, resisting torsional deformation or bending deformation, so that, to increase the stiffness and strength of the torsion beam 3, a reinforcement, such as a torsion bar 8, must be mounted to the torsion beam 3. However, due to the reinforcement, the plate-type torsion beam 3 is problematic in that the increased number of elements constitutes the beam 3, complicates the production process of the beam 3, and increases the weight of a final product.
To solve the problem of the plate-type torsion beam 3, a suspension having a tubular torsion beam has been used in recent years. An example of suspensions having conventional tubular torsion beams is illustrated in FIG. 2. As shown in FIG. 2, a bush sleeve 1, a trailing arm 2, a spring seat 4, a damper bracket 5, a wheel carrier 6 and a spindle plate 7 are used as basic elements constituting a conventional tubular torsion beam suspension.
The tubular torsion beam 10 of the suspension is produced through pressure-forming using a tubular steel member having a circular cross-section. During the pressure-forming, the tubular steel member is shaped into a torsion beam having a cross-section varying along the entire length thereof. The tubular torsion beam 10 comprises opposite ends 11, which have a closed cross-section, such as a triangular, rectangular or circular cross-section, and are mounted to respective trailing arms 2 of the suspension, a middle portion 13, in which a first semicircular surface part 13a is deformed so as to be in close contact with a second semicircular surface part 13b such that they form a V-shaped cross-section, which is open to one side, and a transitional portion 12, the size of the cross-section of which continuously varies and executes a natural transition from the middle portion 13 to each of the opposite ends 11. Described in detail, the middle portion 13 has a small-sized closed cross-section at each end of the V-shaped cross-section. However, because most of the first semi-circular surface 13a is in close contact with most of the second semicircular surface 13b, the middle portion 13 is regarded as a part having an open cross-section.
In FIG. 2, each of the opposite ends 11 is illustrated as having a closed rectangular cross-section with rounded corners. However, it should be understood that the cross-section of the opposite ends 11 is not limited to the rounded rectangular cross-section, but may be configured to have some other closed cross-section, such as a triangular, angled rectangular or circular cross-section, according to the type of vehicle. When the tubular torsion beam 10 having the above-mentioned construction is compared to the plate-type torsion beam 3 having only an open cross-section, the tubular torsion beam 10 has higher stiffness and higher strength, resisting torsion and bending. Thus, the tubular torsion beam 10 may be used without additional reinforcement.
As described above, the tubular torsion beam 10 is produced through shaping such that the torsion beam 10 has a cross-section continuously varying along the entire length thereof. To produce such a tubular torsion beam in the prior art, conventional pressing or hydroforming has been used. An example of conventional pressing techniques is disclosed in Korean Patent No. 554310. The pressing technique disclosed in Korean Patent No. 554310 will be described hereinbelow with reference to FIG. 3.
To produce such a tubular torsion beam through conventional pressing, first, a tubular steel member 20 is placed between upper and lower molds 21 and 22, which have specified shaping surfaces configured to shape opposite ends having a closed cross-section, a transitional portion having a varying cross-section, and a middle portion having a V-shaped open cross-section. After placing the steel member between the two molds, upper and lower pad molds 23 and 24 are actuated so as to shape opposite ends having closed cross-sections through pressing [FIG. 3(a)]. Thereafter, cylinder actuators 26 are operated so as to insert left and right cores 27 into respective opposite ends of the tubular steel member. After the insertion of the cores, the upper and lower molds 21 and 22 are actuated so as to shape a transitional portion and a middle portion through pressing, thus producing a desired tubular torsion beam [FIG. 3(b)]. Thereafter, the upper mold 21 is lifted upwards prior to removing the tubular torsion beam from the lower mold 22 using a push rod 25.
However, the conventional pressing requires a complex molding technique but nevertheless, fails to realize high processing precision, so that the pressing cannot provide a product having a precise cross-sectional shape or a uniform thickness, thus increasing the defective proportion of products.
In an effort to solve the problems of the conventional pressing, hydroforming has preferably been used in recent years. Korean Patent Laid-open Publication No. 2004-110247 discloses an example of a conventional hydroforming technique. The hydroforming technique disclosed in Korean Patent Laid-open Publication No. 2004-110247 will be described with reference to FIG. 4. As shown in FIG. 4, to produce a tubular torsion beam through hydroforming, first, a tubular steel member is placed on a lower mold 32. Thereafter, upper and lower molds 31 and 32 are actuated in cooperation with two guide molds 33, thus shaping opposite ends having a rectangular closed cross-section through pressure forming [FIG. 4(a),(b)]. After shaping the opposite ends, elliptical axial punches 36, which are attached to respective mandrel units, operated in a lengthwise direction relative to the tubular steel member, are advanced so as to seal the opposite ends of the tubular steel member. After sealing the opposite ends, actuation oil is fed into the tubular steel member through inlet holes formed through central axes of the axial punches 36, thus applying hydraulic pressure to the inner surface of the tubular steel member. Thereafter, upper and lower punches 34 and 35 are actuated so as to shape both a middle portion and transitional portions, thus producing a desired tubular torsion beam 30 through pressure forming [FIG. 4(c)].
In the hydroforming technique, pressure of the actuation oil is evenly and continuously applied to the entire inner surface of the tubular steel member, so that it is possible to precisely control the shape and thickness of a tubular torsion beam, thus remarkably reducing the defective proportion of products in comparison with the conventional pressing techniques. Thus, the technique of producing tubular torsion beams through hydroforming has been actively and variously studied recently.
To realize desired vehicle stability, a highly durable design of tubular torsion beams for suspensions has been required. In the prior art, the design of highly durable tubular torsion beams has concentrated on the use of high strength materials or thick materials as materials for the beams. However, the use of high strength materials reduces work efficiency during hydroforming and the use of thick materials increases the weights of car bodies, thus limiting the design of durable tubular torsion beams.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and is intended to provide a tubular torsion beam for rear suspensions of vehicles, which is produced through hydroforming and has an optimal shape, capable of reinforcing a stress-concentrated portion of the beam, with a cross-section varying along the entire length thereof, thus having improved durability. The present invention is also intended to provide a method of manufacturing the tubular torsion beam.